That's at least in part because the shift from active to passive is more of a secular move than a cyclical one.

"We see the ETF product phenomenon as less of an outgrowth of passive investing and more of an adaption of technology and globalization, which are both squarely secular trends," the note said.

The chart shows the "surprisingly high correlation of 0.97" between ETF AUM growth and the number of worldwide internet users. Correlations, loosely speaking, range between 0 and 1, with a 0 indicating no relationship between two metrics and 1 implying that the two metrics always go hand in hand. A correlation of 0.97 between ETFs and internet penetration, then, implies an incredibly strong relationship between those two trends.